Field of Invention
The present invention relates to an impeller for a centrifugal pump. The present invention relates especially to a novel impeller structure for a centrifugal pump used for feeding both fibrous suspensions and water into a headbox of a fibrous web machine. The centrifugal pump utilizing the impeller of the present invention is, for instance, suitable for pumping fibrous suspensions, i.e. stock for liquid-laid paper, tissue or board making applications and for pumping water or other dilution fluid into the headbox circulation. In general, the impeller of the present invention is especially suitable for all such pumping tasks in the production of fibrous webs that a pulseless or low-pulse impeller is needed. The impeller of the present invention may, in its specific construction, be used to pump also foam-based fibrous web making suspensions or mere foam in foam-laid fibrous web making applications.
Background Information
The production of paper, tissue and board has been based on the use of liquid-laid suspensions for more than a century. In other words, the paper, tissue or board making fibres have been suspended in water as a very dilute suspension, which is introduced on the wire or between the wires of the fibrous web machine via at least one so-called headbox. The at least one headbox receives the suspension from a centrifugal feed pump, a so called headbox feed pump. The present day fibrous web machines set high demands for the headbox feed pump.
From the early times of papermaking the production rates of fibrous web machines have continuously increased such that the volume flows of the dilute suspensions introduced to fibrous web machines are now enormous. Since it has been customary practice that only one feed pump may be used for pumping the entire amount of suspension needed for the paper or board manufacture in one fibrous web machine, the size of the headbox feed pumps have grown. This has been the main reason why, in practice, all headbox feed pumps are now so called double-suction pumps.
Additionally, the ever-increasing demands for higher quality of the end product set high requirements for the pulse levels of the headbox feed pumps. Here, in this application, the term ‘headbox feed pump’ is understood to cover all such pumps in fibrous web production that feed any kind of fluid, fibrous or fibreless, into the headbox or between the headboxes of the fibrous web machine or to some other such position that the pulses originating from the pump may have an adverse effect in the quality of the fibrous web product. It is a known fact that centrifugal pumps, due to their type of operation, create pressure pulses in the fluid they are pumping. The known pulses are, on the one hand, created at the point where the fluid that is rotating along with the impeller in the pump volute casing departs from the volute casing to the pressure outlet duct of the pump. A so-called cutwater is a kind of a tongue that physically cuts a part of the rotating fluid to the outlet duct of the pump. A pressure pulse is created each time an impeller working vane passes the cutwater tongue. The same may also be expressed by the cutwater tongue blocking the flow from a vane passage (open flow passage between subsequent working vanes) to the volute. Thus, the pulse frequency (f) may be calculated by using the formula f=z*n/60, where z is the number of impeller working vanes and n is the rotational speed of the impeller in rpm. For instance, if the number of working vanes is 6 and the rotational speed 1200 rpm, the pulse frequency f=120 Hz, or a multitude thereof. On the other hand, it is also a known pulse creation mechanism that when the impeller of a centrifugal pump is running the pressure pulses relate to non-symmetricity of the impeller, whereby such a pulse is notable at a frequency f=n/60, or a multitude thereof.
A few ways to fight the pulsation tendency of a centrifugal pump are known in the prior art. A first, and simple way is to increase the distance between the cutwater tongue and the impeller working vane. However, as the efficiency ratio is inversely proportional to the distance, the distance is, in practice, nowadays, rather reduced than increased.
Another way was to increase the number of impeller working vanes, since the higher the number of impeller working vanes, the lower the pulse amplitude and the higher the pulse frequency. Again, practical reasons prevent increasing of the number of working vanes too high as the efficiency ratio will be compromised due to reducing open cross-sectional flow area and increasing friction in the vane passages.
A yet further way of lowering the pulsation is inclining the outer or trailing edges of the impeller working vanes in relation to the cutwater tongue. It is easy to understand, as an example, that if both the cutwater tongue and the outer edges of the impeller working vanes are edges that extend in the direction of the impeller or pump axis, the pressure pulse, when a working vane passes the cutwater tongue, is as high as it can be, as the two edges pass each other simultaneously for the full length thereof at a close distance. The same applies to all such constructions that the edge of the cutwater tongue and the trailing edge of the working vane parallel. To prevent this abrupt pulse creation mechanism it has been suggested that the impeller working vanes and/or the cutwater tongue are inclined in relation to axial direction or at least to one another such that the above mentioned edges facing each other are not parallel. As soon as the direction of the outer edge of the impeller working vane differs from that of the cutwater tongue, the length (duration) of the pulse is increased and the magnitude of the pulse is lowered. In other words, the fluid flow from the vane passage to the volute is not blocked suddenly, but it is first, in a way, throttled in a narrowing flow path between the working vane and the cutwater tongue. Thus, it has been suggested, for instance in EP-B1-0515466, to increase and incline the number of impeller working vanes and the working vanes such that there is always one working vane facing the cutwater tongue, whereby there would be, in practice, a continuous pulse at the cutwater tongue.
Practice has shown that in headbox feed applications two types of centrifugal pumps applying the above described principles of constructing an impeller are mainly used. Most often, the headbox feed pumps are of a so called double suction structure, i.e. the impeller of the pump having a single shroud with two sets of identical working vanes on both sides of the shroud, and a casing provided with two identical suction inlets on the opposite axial sides of the impeller and a single pressure outlet for delivering the suspension to the headbox. The impeller has been designed such that the working vanes on one side of the impeller shroud are not opposite the working vanes on the other side of the impeller shroud but exactly in between them, i.e. the working vanes are staggered. Thereby, in a way, the pulse frequency at the circumference of the impeller is doubled. Another way of thinking is that both sides of the impeller shroud create their own series of pulse waves, and since the working vanes of the opposite sides of the shroud are staggered, the peaks of pulse waves created by the working vanes on one side of the shroud meet in the outlet duct with the valleys of the pulse waves created by the vanes on the opposite side of the shroud, whereby the pulse waves dampen one another. The result is, depending on the shape of the pulse waves, pulseless or in the least a low-pulse flow. In view of the pressure pulses the double-suction pump is good, as the pulses (peak-to-peak pulses, i.e. pulses measured from the valley to the peak of a pressure wave) are normally of the order of less than 1000 Pa in ordinary headbox feed applications at the critical frequencies, i.e. fi1=frequency of the impeller=n/60, fi2=2*n/60, f1=frequency of the working vanes=z*n/60 and f2=2*z*n/60. The total number of working vanes in headbox feed pumps is typically 12-14. The frequency range the paper or board machine manufacturers consider as critical is 0-100 Hz, sometimes up to 200 Hz.
However, the double suction pump has a complicated construction, as both the impeller and the casing of the pump are difficult, and costly, to manufacture. When in use the double suction pumps have substantially poor efficiency ratios (of the order of 91%), at least when compared to pumps using end suction or single suction impellers. The reasons for the reduction of the efficiency ratio relate to the complicated suction inlet construction, and the inclination of the working vanes, meaning increased surface area (friction) and narrower flow passages. An additional downside in double suction pumps, especially at lower production rates (partial load), is the tendency of the pump to start switching the flow from one side of the impeller shroud to the opposite side thereof and back (cf. von Karman vortex), which means, in practice, that only one of the impeller sides is working at a time. This means that the number of working vanes communicating efficiently with the cutwater tongue is halved, whereby the pulse frequency is also halved so that the pulses the impeller creates may easily come to the critical range. As to the partial load, it is a fact with modern fibrous web machines that their headbox feed pumps are chosen in view of the maximum thickness or basis weight of the end product, whereby the pumps are running almost always at partial load as the fibrous web producers are seldom producing continuously any product, not to mention the heaviest possible one.
A better option in view of both the costs of manufacture and the efficiency ratio is an end suction or a single-suction centrifugal pump, which is closer to an ordinary centrifugal pump of its construction. However, to be able to provide the impeller with a sufficient number of working vanes, for increasing the pulse frequency for instance, without adding the working vanes on a single face of the impeller shroud (which would reduce the cross-sectional open area of the vane passages, and, as a result, the efficiency ratio, significantly), the impeller includes a partition wall as is disclosed in GB-1468029. The partition wall is arranged between the shroud and the front edges of the impeller working vanes such that the working vanes are divided in the flow direction of the fluid to be pumped into two substantially equally wide working vanes. However, to increase the pulse frequency the working vanes on opposite sides of the partition wall are circumferentially staggered, i.e. have been positioned in exactly the same manner as described above in connection with double suction impellers, i.e. the working vanes on one side of the partition wall are exactly in between the working vanes, i.e. in the middle of each vane passage, on the opposite side of the partition wall. Additionally, the working vanes may be inclined as discussed above to lengthen the duration of the pressure pulse. In view of the pressure pulses the end suction pump with a partitioned impeller is only adequate, as the pulses (peak-to-peak pulses, i.e. pulses measured from the valley to the peak of a pressure wave) are normally of the order of less than 2000 Pa in ordinary headbox feed applications at the critical frequencies, i.e. fi=frequency of the impeller=n/60, fi2=2*n/60, f1=frequency of the working vanes=z*n/60 and f2=2*z*n/60. The number of working vanes is typically 12-14 and the critical frequency range 0-200 Hz. In other words, the end suction pump with a partitioned impeller is able to reach the pulse requirement of less than 2000 Pa set for headbox feed pumps by the fibrous web machine manufacturers.
The substantially high pulse value of a single suction partitioned impeller pump of 2000 Pa is caused by the fact that in partial load (discussed already above in connection with double suction pumps), the impeller half located between the shroud and the partition wall takes care of the pumping and the other half forms a recirculation passage. The recirculation is the utmost indication of the nature of the operation of the partitioned impeller. It is a fact that the partitioned impeller may be designed to work optimally in a single operating point (volume flow and head), when the flows via both sides of the partition wall may be said to be in balance. In every other operating point the flows are more or less out of balance. Accordingly, at least the amplitude of the pulse or pressure waves created by the working vanes on one side of the partition wall are not equal with those created by the working vanes on the opposite side of the partition wall, whereby the flow in the outlet or pressure duct of the pump has pressure pulses higher than those in the optimal operating point. And, naturally, the farther from the optimal operating point the impeller or pump is driven, the higher is the pulsation in the pressure or outlet duct. As to the efficiency ratio of the single end suction pump with partitioned impeller it is somewhat better than that of double suction pumps, but still the inclination of the working vanes, meaning increased surface area (friction) and narrower flow passages, reduces the efficiency. As to the manufacture of the pump, the casing of the end suction pump is clearly easier and cheaper to manufacture than that of the double suction pumps. However, the added partition wall makes the construction of the impeller complicated and costly to manufacture.
In other words, the prior art has, for pumps applied at positions where pressure pulsations are considered problematic, a few suggestions. Firstly, the number of impeller working vanes should be increased either by positioning shorter intermediate working vanes between longer ones on the impeller shroud or by partitioning the impeller by means of its shroud (including both the shroud of the double suction pump and the partition wall of the single suction impeller) to two partitions having first working vanes on one face thereof and second working vanes on the other face thereof, the second working vanes being positioned in staggered fashion in relation to the first working vanes. Secondly, the working vanes should be inclined, i.e. the longitudinal centerline plane of the working vane forming a sharp angle with the front face of the impeller shroud in a plane at right angles to the longitudinal vane axis, for increasing the duration of the pressure pulse created by the working vane when passing the cutwater tongue. In practice, in all headbox feed pumps both suggestions have been taken into use to make sure that the pulse level is low enough at the critical frequencies.